This invention relates to vacuum pumps and in particular to compound vacuum pumps which employ at least one turbo-molecular stage and one molecular drag stage.
There is described in European patent publication number 0 919 726 a vacuum pump comprising first and second turbo-molecular stages and a molecular drag (Holweck) stage. This known pump has a first inlet through which fluid being pumped passes through all the pump stages and a second inlet through which fluid enters the pump between the two turbo-molecular stages and passes only through one turbo-molecular stage and the Holweck stage. The turbo-molecular stage upstream of the second inlet is sized differently from the turbo-molecular stage downstream of the second inlet so that the vacuum pump suits the pressure requirements or pumping capacities of the chambers/systems being evacuated and attached to the first and second inlets respectively.
This known xe2x80x9csplit flowxe2x80x9d turbo-molecular pump facilitates the differential pumping of, for example, two chambers of a scientific instrument. For example, in well known types of mass spectrometer that part of the apparatus known as the detector commonly has to be operated at, for example, 10xe2x88x926 mbar whereas that part known as the analyser has to be operated at a different level of vacuum for example 10xe2x88x923 mbar. The chamber requiring the lower pressure (higher vacuum) is attached to the first inlet so that the fluid being evacuated is subject to all the stages of the pump whilst the chamber requiring the higher pressure (lower vacuum) is attached to the second inlet so that the fluid being evacuated is subject only to the pump stages downstream of the second inlet.
With liquid phase mass spectrometer (MS) systems there is increasingly a demand for higher gas loads to be consumed by the vacuum system. The highest gas load handling capacity is required at the comparatively xe2x80x9chighxe2x80x9d pressure end of the pump. As the gas load diminishes in the subsequent chambers so to the system pressure decreases until eventually it reaches a level acceptable for analysis.
It is an aim of the present invention to provide an improved compound vacuum pump including at least one turbo-molecular stage and downstream therefrom a multi-channel molecular drag stage in which one or more channels of the molecular drag stage are devoted to pumping a high pressure fluid inlet whilst the remainder of the channels are devoted to xe2x80x9cbackxe2x80x9d the turbo-molecular stage.
According to the present invention, a compound vacuum pump comprises at least one turbo-molecular stage and downstream therefrom a multi-channel molecular drag stage, a first inlet through which fluid can pass through the turbo-molecular stage and the molecular drag stage towards a pump outlet, a second inlet through which fluid can enter the pump at a location between the turbo-molecular and the molecular drag stages to pass only through the molecular drag stage towards the pump outlet, in which one or more channels of the molecular drag stage are adapted to communicate directly with the second inlet whilst the remaining channel or channels communicate with the turbo-molecular stage.
Preferably, the multi-channel molecular drag stage is a Holweck stage with a plurality of channels arranged as a plurality of helixes.
In one embodiment, a baffle directs one or more of the helical channels of the Holweck stage towards the second inlet.